The present invention relates to a liquid jetting head enabling to eject liquid in the state of droplet such as an ink jet recording head, a liquid crystal jetting head, a coloring material jetting head, etc., particularly to a head having a vibrator unit provided with a plurality of piezoelectric vibrators.
In the liquid jetting head enabling to eject droplet from a nozzle orifice by generating pressure fluctuation in liquid in a pressure chamber, there is an ink jet recording head discharging ink drops used for an image recording apparatus or the like. Recently, the head is applied for every kind of industrial equipment in view of an advantage that very little liquid can be ejected accurately. For example, the head is applied for a liquid jetting head enabling to eject liquid crystal, a coloring material jetting head enabling to eject coloring material for filter, an electrode material jetting head enabling to eject electrode material becoming an electrode, a bio-organic substance jetting head enabling to eject a bio-organic substance, a micro pipette (a sample jetting head) enabling to eject a very little sample accurately, and so on.
In the various kinds of types of such the liquid jetting head, there is a head having a vibrator unit provided with plural piezoelectric vibrators and discharging ink drop by varying volume of a pressure chamber by deformation of the piezoelectric vibrators. The liquid jetting head is roughly constructed by the vibrator unit, a case having a chamber for storing the vibrator unit, and a channel unit joined to the tip end portion of the case. The channel unit is constructed by: a channel forming substrate having a space to be a pressure chamber and a reservoir; a nozzle plate made of metal formed with plural nozzle orifices; a laminated plate of a metal supporting plate and an elastic film; and an elastic plate having an elastic region. The tip end face of each piezoelectric vibrator is bonded on each island portion formed in the elastic region, and liquid drop is ejected from the nozzle orifice by displacing the island portion by deformation of the piezoelectric vibrator. In this construction, remained adhesive is held at the edge of the supporting plate. For example, overflowing adhesive from gap of the tip end face of the piezoelectric vibrator and the island portion is held at a corner portion between the tip end face of the piezoelectric vibrator and the island portion by surface tension of the adhesive. Such a configuration is disclosed in Japanese Patent Publication No. 10-278263A, for example.
For one type of the vibrator unit in which piezoelectric vibrators are fixed on a fixing plate in a cantilevered manner that free end portions of the vibrators are protruded from one face of the fixing plate, the one face of the fixing plate is bonded to an inner wall of the chamber for accommodating the vibrator unit to be fixed within the case.
In such a construction, if the applied quantity of the adhesive for bonding the vibrator unit to the case is excessive, the remained adhesive overflows from the chamber to the elastic plate side. In this case, although it is considered that overflowing adhesive is held at the edge of the supporting plate, the adhesive overflows easily because quantity enabling to hold is very little. The elastic film is hardened by the overflowing adhesive so as to cause fault such as damage of elasticity thereof.
FIG. 8 shows a configuration in which a concave is provided in the elastic plate to hold an adhesive therein in view of the above situation.
In this example, an adhesive holding concave 2 is formed in the vicinity of an elastic region 5 so as to face a corner portion 1 of an opening of an vibrator chamber, by removing a part of a supporting plate 4 so as to remain an elastic film 3. There is formed a partition 4a between the adhesive holding concave 2 and the elastic region 5. A part of piezoelectric vibrators 6, that is, a dummy vibrator 6a which is not involved in the liquid ejection is bonded to the partition 4a. 
The piezoelectric vibrator 6 is produced by laminating a common electrode and a drive electrode alternately sandwiching a piezoelectric material so as to deform in accordance with potential difference application. For example, a potential of the common electrode is set as the lowest potential near the ground potential and a drive signal is applied to the drive electrode to applied the potential difference. In a normal condition, the drive signal is provided as an intermediate potential so that the potential can be risen or fallen from the intermediate potential as a reference potential.
Incidentally, insulating resistance of the piezoelectric material possibly falls by long use of the piezoelectric vibrator 6. When the insulation fault generates, the drive electrode of the piezoelectric vibrator 6 (driving vibrator 6b) can not keep the intermediate potential so as to fall the lowest potential being potential of the common electrode. In this case, when high potential driving signal is supplied to eject liquid drop, the volume and the speed of expansion and contraction of the piezoelectric vibrator 6b become large inordinately, so that liquid drop is ejected unexpectedly.
In order to prevent such an accidental ejection, it is considered that constant bias potential higher than the lowest potential and lower than intermediate potential is applied to the common electrode. In this case, since the potential of the drive electrode falls to at least the bias potential even if insulation fault generates, the accidental liquid drop ejection can be prevented.
However, when the bias potential is applied to the common electrode, there is probability that the bias potential passes through the tip end face of the piezoelectric vibrator 6 and leaks to a nozzle plate. That is, the supporting plate 4 charges to bias potential through the partition 4a via the dummy vibrator 6a. Further, since an introducing port through which liquid flows is formed at the supporting plate 4, liquid charges to bias potential and also the nozzle plate becomes bias potential as the result in the case of the liquid is conductive. In order to prevent charge of the liquid, the nozzle plate is adjusted to ground potential (frame ground). Therefore, short-circuit possibly occurs when the nozzle plate charges.
It is therefore an object of the present invention to provide a structure of liquid jetting head for preventing leakage of bias potential and preventing fault caused by remained adhesive.
In order to achieve the above object, according to the present invention, there is provided a liquid jetting head, comprising:
an elastic plate, including an elastic film and a conductive supporting plate laminated on the elastic film, the conductive supporting plate being partly removed to form an elastic region in which first island portions and second island portions are remained on the elastic film while being electically insulated from each other;
a head case, formed with a chamber which forms an opening at a first end face joined to the supporting plate; and
a vibrator unit, in which a plurality of piezoelectric vibrators are arranged on a fixing plate, the piezoelectric vibrators including a pair of non-deformable vibrators and deformable vibrators arranged between the non-deformable vibrators, wherein:
the vibrator unit is accommodated within the chamber such that each tip end face of the deformable vibrators is abutted onto one of the first island portions, and each tip end face of the non-deformable vibrators is abutted onto one of the second island portions; and
each of the second island portions define a first recess at a portion of the elastic region where opposes to corners of the opening at the first end face of the head case, to receive an adhesive used to bond the vibrator unit to the head case and overflowed from the opening.
In this configuration, the second island portion is placed on the insulative elastic film so as to be insulated from other members, the bias potential applied to the non-deformable vibrator will not leak through the supporting plate. Accordingly, there can be avoided the charging of liquid ejected from nozzle orifices, and the short-circuit to the frame ground. Further, even if an excessive adhesive is overflowed from the chamber, since the adhesive can be held within the first recess, the adhesive is prevented from overflowing to a part of the elastic region at which the first island portions are provided. Therefore, operation fault of the elastic plate can be avoided.
Preferably, a first gap is formed between an edge of the elastic region and each of the second island portions. A width of the first gap is so determined that the adhesive is held therein due to surface tension of the adhesive.
In this configuration, adhesive can be held in the first recess with a simple structure.
Here, it is preferable that each of the second island portions includes a branch part which defines a second recess communicated with the first recess via the first gap. A second gap is formed between the edge of the elastic region and the branch part. A width of the second gap is so determined that the adhesive is held therein due to surface tension of the adhesive.
In this configuration, even if the adhesive overflowed from the opening is more than the capacity of the first recess, the second recess can receive the excessive adhesive.
Preferably, the supporting plate is comprised of a stainless steel plate, and the head case is comprised of an insulative material.
In this case, heat radiating property and electrical insulation can be attained in the supporting plate.
Alternatively, it is preferable that the fixing plate is comprised of an insulative material, and the head case is comprised of a conductive material.
In this case, the design freedom is improved because the head case can be used as a ground electrode.